1. Field of Invention
This invention relates to adhesives. More particularly, this invention relates to two-part room-temperature curing methacrylate based adhesives that are used to bond a wide variety of materials, including thermoset plastics, thermoplastics, metals, wood, ceramics and other materials and combinations of materials. This invention involves a significant improvement in the ability of adhesives to bond certain difficult-to-bond composite materials with minimum required surface preparation. Another feature of this invention is the high degree of elastic behavior of the cured adhesives and the ability of the cured adhesive materials to retain a high proportion of their elastic behavior after exposure to elevated temperatures or long-term aging.
2. Background Art
The three common classes of two-part room temperature curing reactive adhesives are epoxies, polyurethanes, and acrylics. The discussion of these prior art adhesives and the inventive adhesives emphasizes structural applications, wherein a very strong bond is achieved between two structural members of an assembly, and the bond is often strong enough to cause failure of the material before the assembly fails. However, all of these adhesive materials, can be and are used to advantage in less demanding applications, as well, in which case one or more of the advantages of the particular adhesive fulfills a specific bonding requirement.
Epoxy adhesives, which are the earliest, best known and among the most common structural adhesives in general use, consist of an epoxy resin adhesive component and an amine, polyamide, or combined amine and polyamide hardener components. Faster curing epoxies can be formulated with polymercaptan hardeners that are generally used in combination with polyamide and amine hardeners.
Polyurethane adhesives generally consist of an isocyanate-terminated polyol and a hardener or curative component that consists of a polyol or amine or a combination of polyols and amines.
The epoxy and polyurethane adhesives cure upon mixing when the hardener component reacts with the epoxy or polyurethane resin component in an addition polymerization process.
Methacrylate or acrylic adhesives that are used in the same applications as epoxies and polyurethanes generally consist of a polymer-in-monomer solution of an elastomer or thermoplastic resin or a combination thereof in a monomer such as methyl methacrylate. Hardening is achieved when a combination of a peroxide and an amine is introduced into the polymer-in-monomer mixture to initiate a free-radical curing reaction. Generally, the adhesive component contains either the amine or peroxide component and the co-reactive peroxide or amine component is mixed with the adhesive just prior to bonding.
Each of the three common reactive adhesive classes has characteristic advantages and disadvantages. For example, epoxies tend to be characterized as safe and relatively easy to mix and apply, but tend to be somewhat rigid and sensitive to cleanliness of the surface to be bonded. Polyurethanes are generally considered to be much more flexible and elastic, but also suffer from sensitivity to surface contamination, moisture and humidity. Both of these adhesive types have the limitation that fast-curing products tend to have very short open working time after mixing, and products with more acceptable open time have very long cure times. This limitation is imposed by the linear reaction mechanism that is characteristic of the addition polymerization reaction by which they cure.
In terms of the characteristics of the cured adhesive and resulting bond, epoxies are considered to be very strong because of their high modulus or rigidity and resulting high lap shear strength. They are generally recommended for bonding metals because of their affinity for metal surfaces and high shear strengths. However, their rigid nature limits their usefulness in applications that require flexibility in the adhesive bond. Epoxies also have limited ability to bond thermoplastic materials.
Polyurethanes are generally much more elastic, tough and flexible than epoxies. Elasticity, toughness and flexibility are beneficial when adhesive bonds are subjected to peeling or impact forces, and when bonds and bonded assemblies are subjected to dynamic fatigue stresses. However, polyurethanes are not as useful as epoxies for bonding metals, and are generally more suitable for bonding plastic materials in applications that are subjected to bending and impact stresses.
Two-part acrylic or methacrylate adhesives overcome two of the major drawbacks of the epoxies and polyurethanes. They are much more tolerant of unclean or unprepared surfaces, and they have a much more favorable cure profile in terms of open working time and cure rate. In addition, they exhibit equal or better affinity for metal and plastic surfaces than either epoxies or polyurethanes. However, some materials, in particular certain composite materials, are difficult to bond in the xe2x80x9cas receivedxe2x80x9d condition. Specific examples include certain gel coats, which are highly crosslinked and inert polyester compounds that form the outer or xe2x80x9cshowxe2x80x9d surface of fiberglass reinforced polyester (FRP) composite materials used to fabricate boats and other structures exposed to outdoor weathering.
Other examples are closed molded polyester composites, which are materials formed by processes other than the open molded processes used to produce FRP composite structures. Examples of closed molding processes and materials are sheet molding compounds (SMC), resin transfer molded (RTM) composites and pultruded composites.
The essential issues with closed molded processes and products are (1) these processes produce polyester composite articles with reduced emission of and worker exposure to the styrene component in polyester resins and are rapidly replacing open molded processes, and (2) these materials are generally characterized by resistance to the solvating effect of the methacrylate monomers that normally soften or penetrate the bonding surface prior to hardening of the adhesive. In addition, many of these materials use processing aids to provide smooth surfaces for painting. These materials can also interfere with the bonding process.
Other materials are used to facilitate release from the molds used to fabricate parts from them. Such materials are often added directly to the molding compound, in which case they are referred to as xe2x80x9cinternalxe2x80x9d mold releases. Other materials may be sprayed on to the mold surface prior to molding. These materials are referred to as xe2x80x9cexternalxe2x80x9d mold releases. All of these processing aids can interfere with the formation of strong adhesive bonds.
The problems experienced in bonding these materials with prior art methacrylate-based adhesives, as well as the additional and undesirable processing steps required to use them, including grit-blasting, sanding, solvent wiping and priming are described in detail in U.S. Pat. No. 3,838,093, which is discussed in further detail below.
Epoxy adhesives based on standard DGEBA (diglycidyl ether of bisphenol-A) resins, cured with hardeners based on combinations of amines, polyamides and other additives used to impart specific properties, have effectively been used to bond some closed molded composite materials. However these adhesives do not completely cure at room temperature, and generally require thermal post-curing to develop their full physical strength.
Recent developments in polyurethane adhesive technology have been directed toward improving adhesion to these composite materials as disclosed, for example, in U.S. Pat. Nos. 5,340,901 and 5,548,056. However, as with epoxy adhesives, these materials often require thermal post curing. Even though polyurethanes do ultimately develop their full physical strength at ambient temperatures, such post curing may be required to meet process speed requirements or to develop full, reliable or reproducible adhesion to the composite surface, or both. In some cases, solvent-based primers are used to develop adhesion at ambient temperatures, but this is undesirable for environmental and health reasons.
Yet another problem with the epoxy and polyurethane-based adhesives is their limited ability to bond to open-molded composite surfaces. Open-molded composite articles are fabricated by using a combination of spraying and rolling processes that combine woven and chopped fiberglass with polyester laminating resins in open molds. A mold in the shape of the article is first sprayed with a gel or outer coat of a highly crosslinked, pigmented resin that creates a smooth, shiny exterior or xe2x80x9cshowxe2x80x9d surface. The laminating resin and glass are then applied together in successive applications until the desired part thickness is achieved. It is desirable to be able to de-mold the molded article as quickly as possible and move it on to the assembly area. At this point, and for several hours thereafter, the resin is not completely cured and is referred to as xe2x80x9cgreenxe2x80x9d as the final stage of the polymerization process proceeds. In this state, the exposed or xe2x80x9crawxe2x80x9d resin surface is difficult or impossible to bond with conventional epoxy or polyurethane adhesives.
Thus, while epoxies and polyurethanes are sometimes capable of bonding the gel coat or show surfaces of these resin structures, in most cases it is necessary to bond the raw surface to itself or to the gel coat surface. Methacrylate adhesives have been increasing in popularity and usage because of their ability to bond the raw fiberglass surfaces, even in the green state. However, as improvements and changes have been made in the composition of the gel coat materials in recent years, the ability of the methacrylates to bond them, especially in the xe2x80x9cas receivedxe2x80x9d, condition, has become less predictable.
A significant characteristic of some of the more recent acrylics or methacrylates is elasticity, toughness and flexibility that is greater than that of epoxies and even approaches that of the polyurethanes. However, not all of the methacrylate adhesives exhibit such elasticity, toughness and flexibility initially, and many of those that do often fail to retain these properties over a long period of time or when heated to elevated temperatures. Such reduction in elasticity can be demonstrated by comparing the bulk stress-strain properties of films prepared from the compositions which have not been exposed to elevated temperatures with similar films that have been subjected to brief or prolonged exposure at various temperatures. Loss of elasticity that occurs upon brief exposure at elevated temperatures may be the result of a continuation of the curing process, or a xe2x80x9cpost curingxe2x80x9d process. It is also believed that certain physical changes in the phase distribution of the glassy and rubbery components or domains can occur in the cured composition when it is heated to or above its glass transition temperature or Tg. Loss of elasticity that occurs upon prolonged exposure to elevated temperature can also be the result of either the post curing or physical processes described above or the chemical degradation because of oxidative or other thermally induced reactions that adversely affect the polymer structure.
The improvements of this invention are primarily directed toward changes that occur as a result of post curing or physical changes after relatively brief exposures to elevated temperatures or longer exposures to ambient temperatures. Resistance to oxidative or other thermal degradation processes is subject to other chemical formulating principles well known to those skilled in the art.
Practical manifestations of these phenomena include the potential loss of the ability of adhesive bonds to resist peeling or impact forces as the bonded assembly ages, or a reduction in the elasticity or increase in hardness of the cured composition in the center portion of a thick cross section. The latter is believed to occur when the exothermal heat generated by the polymerizing composition raises the temperature to a level that approaches or exceeds the Tg of the cured composition. Whatever the cause of the physical changes that occur during or after the initial curing phase of the adhesive, the ultimate physical and elastic characteristics of the adhesive can generally be predicted by a brief exposure of the material in question at an elevated temperature. Typical thermal exposures for this purpose are from 30 minutes to a few hours at temperatures ranging from about 70 degrees centigrade to about 100 degrees centigrade.
As the use of adhesives increases in the fabrication of composite structures, design engineers are increasingly aware of the need to reliably predict the physical characteristics of the adhesives, which become an integral part of the structure. In this regard, adhesives are generally characterized by their tensile strength, modulus or stiffness, and elastic properties. In some cases, a stiffer adhesive is desired in order to provide a high degree of load bearing capability in the bonded joint. In other cases, a more flexible or elastic joint may be required in order to resist shock, vibration and fatigue loads. In civil engineering applications, such as highway bridge construction, a somewhat stiffer bond may be desirable. However, it is important that the adhesive also have a predictable degree of elastic behavior in order to withstand the bond stresses that occur as a result of thermal cycling and the resulting differential expansion of the bonded components. It is also necessary to withstand the cyclic loading of the structure imposed by vehicular traffic on the bridge. In extreme cases, seismic loads on civil engineering structures demand the utmost in stress to failure capability of the adhesive and the resulting joint.
In the fabrication of boats, on the other hand, more flexible adhesives are often desirable. An example is the bonding of stringers or liners in the structural fabrication of the boat hull. In this application, there can be a combination of severe shear, peel, and shock loading of the bonds when the boat is operated at high speeds over rough or choppy water. Flexible adhesives can provide very durable joints by resisting the peel and shock loads imposed on the bond and by damping the energy transferred to the joint as it undergoes cyclic loading.
In all of these cases, it is imperative that the adhesive be capable of retaining the physical characteristics, especially the ability to withstand shock and fatigue loads, during the lifetime of the structure. It is further important that the components of the adhesive can be adjusted to provide the desired degree of stiffness and flexibility for a variety of applications.
It is clear from the discussion above that there is a need for adhesives that will reliably and predictably bond a wide variety of composite surfaces in the as received condition, rapidly and without the application of heat to complete the cure or develop full adhesive bond strength. It is also desirable for such adhesives to bond other structural materials such as metal, thermoplastics, wood, etc. It is further desirable that such adhesives possess a high and predictable degree of elasticity and retain their elasticity when exposed to elevated temperatures during the curing process or in service.
The benefits derived from the improvements of this invention apply to structural adhesive bonding applications. However, the compositions disclosed herein may also be useful in a number of other applications for which epoxy, polyurethane, methacrylate and polyester resins are employed. One such application is coatings. A specific example of a coating is the driving surface of bridge decks, including FRP or composite bridge decks which may be fabricated and assembled using adhesives, including the adhesive compositions of the present invention. Such coatings are often referred to as bridge deck overlays.
FRP or composite bridges and bridge decks have been developed to replace traditional steel and concrete structures for a number of reasons, including their resistance to rust and decay in severe climates. Their light weight and high strength relative to steel and concrete structures make them especially useful for reconstructing aged, deteriorated bridge structures. Composite decks can be used to replace the existing concrete and steel deck of a deteriorated structure without replacing the entire structure. The light weight and equivalent or superior load-bearing capability of the composite deck allows the bridge to support the same traffic loads as the existing structure without replacing other deteriorated structural support members of the bridge.
Reconstructed bridges, as well as new and replacement bridges can be installed more quickly and with less traffic disruption than traditional bridges. Other benefits of this concept are too numerous to mention and beyond the scope of this invention. However, a common problem with these applications is the final step of replacing the pavement or driving surface of the bridge deck.
Existing bridge resurfacing materials are generally composed of polymer latex modified concrete or an aggregate composition that uses an epoxy resin or polyester resin as a binder for the aggregate and adhesive to secure the surface to the bridge deck. These materials were originally developed and have been used for resurfacing traditional concrete or asphalt bridge decks. Application of these materials to composite bridge decks has been less than satisfactory for a number of reasons, including mismatched coefficient of thermal expansion relative to the composite deck, insufficient toughness and flexibility, poor or marginal adhesion, and complexity in mixing and application.
The benefits of the present invention for adhesives, namely elasticity and toughness, and the retention of elasticity and toughness, combined with the ability to bond a number of surfaces, including difficult to bond composites, are useful for replacement of the driving surface of the bridge as well. Their toughness, flexibility and resistance to cracking also provide potential benefits for overlay coatings for restoration of existing concrete and asphalt bridge deck surfaces. In this case, the coating can perform both as a traffic wear surface and as a sealant to prevent intrusion of moisture, salt and other damaging elements that can damage the concrete and metal bridge structure beneath the pavement. For this application, it is imperative that the coating be resistant to cracking or any other loss of integrity that allows moisture or damaging agents such as deicing chemicals, oils or fuels to penetrate the coating. Such penetration can eventually lead to disbondment of the overlay and or damage to the structural components of the bridge deck and supporting structures.
U.S. Pat. No. 3,333,025 discloses improvements in the adhesive properties of polymerizable adhesives based on mixtures of methyl methacrylate monomer, styrene monomer, polychloroprene, and optionally an unsaturated polyester resin.
U.S. Pat. No. 3,838,093 describes problems associated with the bonding of fiberglass reinforced polyester (FRP) substrates with adhesives, including the adhesives of the ""025 patent. It further discloses compositions of primers based on isocyanate and polyol components as primers, wherein the primers require volatile organic solvents in order to be effectively applied. It further discloses the requirement to cure the primer by allowing it to stand at ambient temperatures for up to 72 hours, or by baking the primed substrate in an oven at 200-280 degrees F.
U.S. Pat. No. 3,890,407 discloses methacrylate adhesives with improved adhesive properties comprising mixtures of chlorosulfonated polyethylene (CSPE) in methyl methacrylate (MMA) monomer. Among the compositions disclosed are mixtures of Hypalon 20 and Hypalon 30 CSPE in MMA with other additives to complete the adhesive formulations. Among the improvements cited are increased speed of cure, improved adhesion to unclean or unprepared surfaces, and high bond strength.
U.S. Pat. No. 4,126,504 discloses methacrylate monomer based adhesives containing a variety of polymers, including polychloroprene, chlorosulfonated polyethylene, and butadiene/acrylonitrile. It suggests that mixtures of such polymers may be employed, but does not cite or claim specific mixtures or combinations of polymers or suggest or disclose specific advantages obtainable through the use of such mixtures. In particular, it does not suggest mixtures of polychloroprene or chlorinated polyethylene polymers with butadiene-acrylonitrile polymers.
U.S. Pat. No. 5,206,288 discloses methacrylate adhesives based on mixtures of a number of elastomers blended individually with a core-shell impact modifier. These adhesives exhibit a high degree of toughness and flexibility, especially at low temperatures. Polychloroprene and butadiene-acrylonitrile elastomers are disclosed individually in combination with the core-shell impact modifiers, but there is no suggestion of employing blends of these elastomers in combination with the impact modifiers.
It has now been discovered that unique and highly beneficial adhesive characteristics can be achieved by blending chlorinated polymers such as polychloroprene, chlorinated polyethylene and chlorosulfonated polyethylene with butadiene-acrylonitrile elastomers and methacrylate monomers and free-radical catalysts to form polymerizable methacrylate adhesives. Such adhesives display excellent adhesion to difficult-to-bond composite surfaces, without the need for extensive surface preparation. Moreover, the adhesives exhibit a high degree of elasticity and retain their elasticity following exposure to heat.
The adhesives of the invention encompass a combination of about 1 to about 45 percent by weight, preferably from about 2 percent to about 35 percent by weight, most preferably from about 5 percent to about 25 percent by weight of a chlorinated polymer, preferably a chlorinated elastomer polymer, about 1 percent to about 45 percent, preferably from about 1 percent to about 25 percent, and most preferably about 1 percent to about 20 weight percent by weight of a nitrile elastomer, a rubber-modified acrylonitrile copolymer or mixtures thereof, and about 25 percent to about 90 percent by weight of an alkyl acrylate or methacrylate monomer, preferably a C1 to C5 alkyl acrylate or methacrylate monomer.
In a further preferred embodiment, the adhesives of the invention encompass a combination of about 1 percent to about 45 percent, preferably from about 5 percent to about 25 percent, of a chlorinated elastomer polymer, about 1 percent to about 45 percent, preferably from about 1 percent to about 25 percent of a nitrile elastomer, from about 0.01 to about 30 percent, preferably from about 0.01 to about 20 percent of a core-shell impact modifier in combination with a methacrylate monomer or a mixture of methacrylate monomers. The preferred monomers are C1 to C5 alkyl acrylate or methacrylate monomers which constitute from about 25 percent to about 90 percent by weight, preferably from about 40 percent to about 90 percent by weight, and most preferably, from about 50 percent to about 85 percent by weight of the composition. The most preferred monomer is methyl methacrylate monomer. Additional useful monomers are C6 or higher methacrylate monomers which can constitute from 0.01 to about 25 percent, and preferably from 0.01 to about 15 percent of the composition. The compositions may also contain from 0 to about 15 percent, preferably from 0.01 to about 10 percent of a polymerizable organic acid monomer. The compositions are cured with a free radical generating catalyst system.
The preferred monomers of the invention are alkyl methacrylate monomers, but other monomers such as, for example, acrylate monomers may also be used to advantage in the compositions. Similarly, methacrylate or acrylate monomers other than alkyl methacrylates may be used in varying proportions to impart specific properties, as may be other compatible, polymerizable vinyl monomers including styrene and monomers derived from styrene.
The adhesive compositions of the inventions are characterized by their ability to bond a variety of composite materials, including certain difficult to bond composites with or without preparation of the surface prior to bonding, as well as a variety of other materials alone or in combination. The adhesives are further characterized by their high degree of elasticity and their ability to retain their elastic behavior after exposure to elevated temperatures or long term aging under ambient conditions.
The essential feature of this invention is the use of a combination of a chlorinated polymer, preferably a chlorinated elastomeric polymer, and a copolymer of acrylonitrile with butadiene or isoprene, or a combination of butadiene and isoprene, as the elastomeric component of a methacrylate-based adhesive.
The preferred chlorinated elastomer polymers of this invention are polychloroprene, chlorosulfonated polyethylene, chlorinated polyethylene, other chlorinated hydrocarbon polymers, chlorinated rubber, epichlorohydrin polymers, or other polymers containing from about 20 to about 55 percent chlorine, and preferably from about 25 to about 45 percent chlorine. Examples of preferred chlorosulfonated polyethylene polymers are Hypalon 20, Hypalon 30, Hypalon 40, and Hypalon 48. Examples of preferred chlorinated polyethylene polymers are Tyrin 3615, Tyrin 4211, and Tyrin 4215. Examples of preferred polychloroprene polymers are Neoprene AD-5, AD-10, AD-20, and other adhesive and non-adhesive grades of polychloroprene polymers. All of these preferred chlorinated polymers are available form DuPont Dow Elastomers among other commercial manufacturers.
Chlorinated polymers such as polyvinyl chloride (PVC) or vinyl chloride copolymers such as vinyl chloride/vinyl acetate copolymers may also be employed. Unmodified polyvinyl chloride is not readily soluble in the methacrylate monomers of this invention, but may be dispersed under conditions of high shear. The copolymers, on the other hand are readily soluble. Another useful method of incorporation of polyvinyl chloride polymers is the addition of nitrile elastomer/PVC blends. Such blends, which typically contain from about 20 percent to about 80 percent, and preferably from about 30 percent to about 70 percent nitrile elastomer are sold by Zeon Chemicals under the trade name Nipol. Specific examples are Nipol P-70, a 70/30 nitrile-PVC blend, and Nipol 503 F1, a 50/50 blend.
The nitrile elastomer polymers of the instant invention are butadiene-acrylonitrile or butadiene-isoprene-acrylonitrile polymers with an acrylonitrile content from about 15 to about 50 percent, preferably from about 18 to about 45 percent. The remainder of the nitrile polymer backbone is typically butadiene or a combination of butadiene and isoprene. The glass transition temperatures (Tg) of the polymers range from about xe2x88x9250 to about 0 degrees C., and preferably from about xe2x88x9240 degrees C. to about xe2x88x9210 degrees C. Examples of such polymers include polymers available from Zeon Corporation, such as Nipol DN 401LL, Nipol DN 1201L, Nipol 1401LG, Nipol 1472 and Nipol DN 4555.
Certain rubber-modified acrylonitrile copolymers including acrylonitrile-butadiene-styrene (ABS) and methacrylate-acrylonitrile-butadiene-styrene (MABS) solution or molding grade resins are also useful components of this invention, but they usually are most effective when used in combination with butadiene-acrylonitrile or butadiene-isoprene-acrylonitrile elastomers. Solution grade or molding grade resins that are useful in conjunction with the chlorinated and nitrile elastomers of this invention typically contain from about 5 to about 50 percent, preferably from about 5 to about 30 percent butadiene, the remainder of the polymer primarily comprising styrene and acrylonitrile, or styrene, acrylonitrile and methyl methacrylate. Other styrenic or vinyl monomers may also be employed. The range of compositions of the polymers and a description of the various processes used to prepare them are described in U.S. Pat. No. 4,233,418.
One useful ABS resin is Blendex ADG-21 supplied by General Electric. It is a solution grade resin sold for the manufacture of solvent cements for ABS pipe. It contains about 35.5 percent acrylonitrile, about 7.5 percent butadiene and about 60 percent styrene. A useful MABS resin Terlux 2812 TR from BASF, a specialty clear grade sold for molding applications requiring a high degree of clarity and retention of physical properties. It has a density of 1.08 g/cc, tensile strength of 5,988 psi and tensile modulus of 278,200 psi. It is evident from the range of ABS and MABS polymers that are useful in this invention that these examples should not be limiting. For example, ABS and MABS solution and molding grade resins typically contain a substantial portion of free styrene acrylonitrile copolymer (SAN) copolymer, and thus the SAN copolymer is an effective component as well.
When used, these polymers are incorporated in amounts from 0.01 to about 30 percent, preferably from about 2 to about 25 percent of the adhesive composition.
Other polymeric materials may be used to advantage in the compositions of this invention. For example, core-shell impact modifiers may be used to impart desirable non-sagging Theological characteristics, and may also impart other desirable adhesive or mechanical property characteristics. Examples of such polymers are MABS, ABS, or MBS polymer impact modifiers of the core shell type. The MBS polymers are similar to those described in U.S. Pat. No. 4,304,709, which is hereby incorporated by reference. The MBS polymers are generally made by polymerizing methyl methacrylate monomer and styrene in the presence of polybutadiene or a polybutadiene copolymer rubber.
ABS and MABS impact modifiers are made by a similar process wherein the methyl methacrylate monomer is replaced by acrylonitrile or a combination of acrylonitrile and methyl methacrylate monomer, respectively. These polymers are described in U.S. Pat. No. 4,513,108 which is hereby incorporated by reference. An example of an ABS impact modifier is Blendex 338 produced by General Electric Company.
Another optional feature of this invention is the use of a mixed acrylate or methacrylate monomer composition. While the primary component of the monomers will be methyl or ethyl methacrylate or other acrylates or methacrylates with five or less carbon atoms in the alcohol portion, these monomers may be combined with longer chained monomers such as the C6 and greater acrylates or methacrylates. The most preferred longer chain acrylates and methacrylates are dodecyl (C12, also known as lauryl) and stearyl methacrylates, may be based on linear aliphatic alcohols. Other preferred monomers of this group are n-hexyl n-octyl, n-decyl, octadecyl, and other similar monomers with an aliphatic alcohol group of up to about 24 carbon atoms. The relative amounts of the short chain and long chain acrylate or methacrylate monomers that are used in the compositions depend upon the particular end use.
The longer chain methacrylate monomers generally increase the flexibility and toughness of the cured adhesive composition. They can also affect the adhesive characteristics of the composition by increasing or decreasing the affinity of the adhesive composition toward a particular substrate or group of substrates. In general, higher levels of methyl methacrylate provide higher modulus and stiffness and reduced toughness and flexibility. The amount of longer chain monomer is selected to achieve the desired level of flexibility and toughness, without unduly reducing stiffness and modulus. Similarly, the amount of longer chain methacrylate monomer is selected in such a way as to achieve the desired adhesion characteristics without unduly and negatively affecting other desirable adhesive characteristics.
Another function of the long chain acrylic or methacrylate monomer is to extend the open time of the adhesive by reducing the volatility of the monomer mixture. To further extend the open time, natural or synthetic waxes may be added in amounts ranging from about 0.1 to 2 percent or more. The addition of such waxes further extends open time by acting as a barrier to reduce the rate of evaporation of monomer from the surface of the wet adhesive layer.
The preferred monomers of the invention are alkyl methacrylate monomers. However, acrylate monomers may also be used to advantage in the compositions. Similarly, methacrylate or acrylate monomers other than alkyl methacrylates may be used in varying proportions to impart specific properties. Examples of other useful monomers are hydroxyl substituted monomers such as hydroxyethyl and hydroxypropyl methacrylate and acrylate, tetrahydrofurfuryl acrylate and methacrylate, cyclohexyl methacrylate and methoxyethyl methacrylate. Difunctional crosslinking monomers and oligomers, including those derived from epoxy and polyurethane backbones can also be used to advantage in the compositions. The general selection of available monomers and oligomers is well known to those in the art and is reviewed in U.S. Pat. No. 5,935,711.
When used, the monomers, other than the C1-C5 alkyl acrylates and methacrylates, are added in amounts ranging from about 0.01 to about 25 percent of the polymer and monomer composition, preferably from about 0.5 to about 15 percent.
In order to further promote adhesion, especially to metals, the presence of a polymerizable acid such as methacrylic acid is preferred, in amounts ranging from about 0.01 to about 15 percent based on the total composition. Other useful polymerizable acids are acrylic acid, maleic acid, itaconic acid, and other copolymerizable unsaturated acids of this type that are well known to those skilled in the art.
In order to control the viscosity of the adhesive, various viscosity control agents such as organoclays, fumed silica or the like may be added in amounts ranging from about 0.1 to about 10 percent based on the system weight. Additional fillers may be added in significantly larger amounts to reduce the cost of the adhesive or to modify certain physical properties such as shrinkage and exotherm characteristics. In this case, quantity of the filler or extender would be considered separately as an additive to the base polymer and monomer composition as described above.
Common particulate fillers or extenders such as clay, talc, calcium carbonate, silica and alumina trihydrate can be added in amounts up to about 50 percent or more of the composition by weight in order to achieve specific economic, application or bonding characteristics. Inorganic or organic microspheres or microballoons may be used to reduce the density and cost of the adhesives, as well as to improve their sanding or finishing characteristics when used as repair materials such as automobile body repair products.
Any number of available and well-known catalysts may be chosen to cause the polymerization and curing of the compositions of the instant invention. The terms used to describe the various components of the curing system (catalysts, initiators, activators, promoters) are often used interchangeably, and thus the terminology used below may differ from other descriptions used in the art.
Generally, the catalyst chosen is a free radical generating catalyst. Examples of these catalysts are benzoyl peroxide, cumene hydroperoxide, tertiary butyl hydroperoxide, dicumyl peroxide, tertiary butyl peroxide acetate, tertiary butyl perbenzoate, ditertiary butyl azodiisobutyronitrile and the like. These free radical producing catalysts are used in amounts of about 0.01 to about 10 weight percent based on the weight of the adhesive composition. Preferably, the catalysts will be used in the amount of about 0.05 to about 5 weight percent.
Other components that promote or enhance the reactivity of the catalysts are initiators or activators and promoters. The initiators or activators are added in the amount of up to about 15 weight percent based on the weight of the adhesive. Preferred amounts are 0.01 to about 5 percent. Initiators and activators, which terms are often used interchangeably, include tertiary amines and aldehyde-amine reaction products. Useful tertiary amines include N,N-dimethylaniline, N,N-diethyltoluidine, N,N-bis(2-hydroxyethyl) toluidine and the like.
Aldehyde-amine reaction products include such compositions as butyraldehyde-aniline and butyraldehyde-butylamine derivatives whose active ingredient is a dihydropyridine (DHP) formed from condensation of three moles of aldehyde with one mole of amine. More recently, DHP-enriched versions of these compositions have been made available. One such material is Reillcat ASY-2, available from Reilly Industries, Inc. This catalyst or initiator system is most often used in combination with a sulfonyl chloride compound and a hydroperoxide as described in U.S. Pat. Nos. 3,890,407 and 4,182,644.
Compositions utilizing chlorosulfonated polyethylene, a hydroperoxide such as cumene hydroperoxide, and DHP as the catalyst/initiator system are generally mixed and applied in one of two ways. One is to include the chlorinated polyethylene and hydroperoxide along with all other adhesive components in a single polymer-in-monomer composition. The DHP can be applied to the bonding surfaces by brushing, rolling or spraying, which was the preferred method disclosed in the above-referenced ""407 patent. The most popular current commercial method of use is to formulate a 1:1 mix ratio adhesive wherein the chlorosulfonated polyethylene and hydroperoxide are included in one polymer-in-monomer solution (the adhesive) and the DHP is added to another formulated polymer-in-monomer solution (the activator). The compositions disclosed in the examples can be utilized in either manner, but for convenience are shown as single solution blends in order to identify the proportions of the formulating components.
A promoter is an organic salt of a transition metal, such as cobalt, nickel, manganese or iron naphthenate, copper octoate, copper acetylacetonate, iron hexoate, or iron propionate. Promoters are used to enhance cure rate. Promoters, whose effect varies greatly from system to system are used in amounts up to about 1-2 weight percent, preferably about 1 part per million to about 0.5 weight percent. Most preferred amounts are from about 5 parts per million to about 0.5 percent by weight.
The most preferred free radical generating systems are (1) a tertiary amine reacting with benzyl peroxide or another peroxide or (2) a DHP derivative in combination with a sulfonyl chloride compound and a hydroperoxide to induce room temperature free radical curing.
The compositions of this invention have been developed primarily to improve the properties of adhesives. However, the improvements thereby discovered make these products more useful than previous products of their class for repair materials, coatings, bulk casting and any number of other applications beyond adhesives.
A specific example is the bridge deck overlay application discussed in the Prior Art. The preferred major components of this invention, namely poly (methyl methacrylate), nitrile polymers and elastomers, and chlorinated polymers and elastomers individually and collectively are inherently resistant to the adverse elements to which such overlays are exposed. Moreover, these components can readily be altered and adjusted in terms of amount and composition to optimize their adhesive, physical and chemical properties, all within the scope of this invention. All of these characteristics and benefits that are important for adhesives, including the ability to bond composites and a variety of other materials without surface preparation, and permanent retention of elastic properties, are applicable to a number of other applications, one example of which is bridge deck overlays.